eye of a naturalist to distinguish them one from another. If we
disregard the small differences which separate these forms and consider
all the species of such groups as modifications of one type, we shall
find that, even among the higher animals, some types have had a
marvellous duration. In the chalk, for example, there is found a fish
belonging to the highest and the most differentiated group of osseous
fishes, which goes by the name of _Beryx_. The remains of that fish are
among the most beautiful and well preserved of the fossils found in our
English chalk. It can be studied anatomically, so far as the hard parts
are concerned, almost as well as if it were a recent fish. But the genus
_Beryx_ is represented, at the present day, by very closely allied
species which are living in the Pacific and Atlantic Oceans. We may go
still farther back. I have already referred to the fact that the
Carboniferous formations, in Europe and in America, contain the remains
of scorpions in an admirable state of preservation, and that those
scorpions are hardly distinguishable from such as now live. I do not
mean to say that they are not different, but close scrutiny is needed in
order to distinguish them from modern scorpions.
More than this. At the very bottom of the Silurian series, in beds which
are by some authorities referred to the Cambrian formation, where the
signs of life begin to fail us--even there, among the few and scanty
animal remains which are discoverable, we find species of molluscous
animals which are so closely allied to existing forms that, at one time,
they were grouped under the same generic name. I refer to the well-known
_Lingula_ of the _Lingula_ flags, lately, in consequence of some slight
differences, placed in the new genus _Lingulella_. Practically, it
belongs to the same great generic group as the _Lingula_, which is to be
found at the present day upon your own shores and those of many other
parts of the world.
The same truth is exemplified if we turn to certain great periods of the
earth's history--as, for example, the Mesozoic epoch. There are groups
of reptiles, such as the _Ichthyosauria_ and the _Plesiosauria_, which
appear shortly after the commencement of this epoch, and they occur in
vast numbers. They disappear with the chalk and, throughout the whole of
the great series of Mesozoic rocks, they present no such modifications
as can safely be considered evidence of progressive modification.
Facts of this kind are undoubtedly fatal to any form of the doctrine of
evolution which postulates the supposition that there is an intrinsic
necessity, on the part of animal forms which have once come into
existence, to undergo continual modification; and they are as distinctly
opposed to any view which involves the belief, that such modification as
may occur, must take place, at the same rate, in all the different types
of animal or vegetable life. The facts, as I have placed them before
you, obviously directly contradict any form of the hypothesis of
evolution which stands in need of these two postulates.
But, one great service that has been rendered by Mr. Darwin to the
doctrine of evolution in general is this: he has shown that there are
two chief factors in the process of evolution: one of them is the
tendency to vary, the existence of which in all living forms may be
proved by observation; the other is the influence of surrounding
conditions upon what I may call the parent form and the variations which
are thus evolved from it. The cause of the production of variations is a
matter not at all properly understood at present. Whether variation
depends upon some intricate machinery--if I may use the phrase--of the
living organism itself, or whether it arises through the influence of
conditions upon that form, is not certain, and the question may, for the
present, be left open. But the important point is that, granting the
existence of the tendency to the production of variations; then, whether
the variations which are produced shall survive and supplant the parent,
or whether the parent form shall survive and supplant the variations, is
a matter which depends entirely on those conditions which give rise to
the struggle for existence. If the surrounding conditions are such that
the parent form is more competent to deal with them and flourish in
them, than the derived forms, then, in the struggle for existence, the
parent form will maintain itself and the derived forms will be
exterminated. But if, on the contrary, the conditions are such as to be
more favourable to a derived than to the parent form, the parent form
will be extirpated and the derived form will take its place. In the
first case, there will be no progression, no change of structure,
through any imaginable series of ages; in the second place, there will
be modification and change of form.
Thus the existence of these persistent types, as I have termed them, is
no real obstacle in the way of the theory of evolution. Take the case of
the scorpions to which I have just referred. No doubt, since the
Carboniferous epoch, conditions have always obtained, such as existed
when the scorpions of that epoch flourished; conditions in which
scorpions find themselves better off, more competent to deal with the
difficulties in their way, than any variation from the scorpion type
which they may have produced; and, for that reason, the scorpion type
has persisted, and has not been supplanted by any other form. And there
is no reason, in the nature of things, why, as long as this world
exists, if there be conditions more favourable to scorpions than to any
variation which may arise from them, these forms of life should not
persist.
Therefore, the stock objection to the hypothesis of evolution, based on
the long duration of certain animal and vegetable types, is no objection
at all. The facts of this character--and they are numerous--belong to
that class of evidence which I have called indifferent. That is to say,
they may afford no direct support to the doctrine of evolution, but they
are capable of being interpreted in perfect consistency with it.
There is another order of facts belonging to the class of negative or
indifferent evidence. The great group of Lizards, which abound in the
present world, extends through the whole series of formations as far
back as the Permian, or latest Palæozoic, epoch. These Permian lizards
differ astonishingly little from the lizards which exist at the present
day. Comparing the amount of the differences between them and modern
lizards, with the prodigious lapse of time between the Permian epoch and
the present age, it may be said that the amount of change is
insignificant. But, when we carry our researches farther back in time,
we find no trace of lizards, nor of any true reptile whatever, in the
whole mass of formations beneath the Permian.
Now, it is perfectly clear that if our palæontological collections are
to be taken, even approximately, as an adequate representation of all
the forms of animals and plants that have ever lived; and if the record
furnished by the known series of beds of stratified rock, covers the
whole series of events which constitute the history of life on the
globe, such a fact as this directly contravenes the hypothesis of
evolution; because this hypothesis postulates that the existence of
every form must have been preceded by that of some form little different
from it. Here, however, we have to take into consideration that
important truth so well insisted upon by Lyell and by Darwin--the
imperfection of the geological record. It can be demonstrated that the
geological record must be incomplete, that it can only preserve remains
found in certain favourable localities and under particular conditions;
that it must be destroyed by processes of denudation, and obliterated by
processes of metamorphosis. Beds of rock of any thickness, crammed full
of organic remains, may yet, either by the percolation of water through
them, or by the influence of subterranean heat, lose all trace of these
remains, and present the appearance of beds of rock formed under
conditions in which living forms were absent. Such metamorphic rocks
occur in formations of all ages; and, in various cases, there are very
good grounds for the belief that they have contained organic remains,
and that those remains have been absolutely obliterated.
I insist upon the defects of the geological record the more because
those who have not attended to these matters are apt to say, "It is all
very well, but when you get into a difficulty with your theory of
evolution, you appeal to the incompleteness and the imperfection of the
geological record;" and I want to make it perfectly clear to you that
this imperfection is a great fact, which must be taken into account in
all our speculations, or we shall constantly be going wrong.
[Illustration: FIG. 2.--TRACKS OF BRONTOZOUM.]
You see the singular series of footmarks, drawn of its natural size in
the large diagram hanging up here (Fig. 2), which I owe to the kindness
of my friend Professor Marsh, with whom I had the opportunity recently
of visiting the precise locality in Massachusetts in which these tracks
occur. I am, therefore, able to give you my own testimony, if needed,
that the diagram accurately represents what we saw. The valley of the
Connecticut is classical ground for the geologist. It contains great
beds of sandstone, covering many square miles, which have evidently
formed a part of an ancient sea-shore, or, it may be, lake-shore. For a
certain period of time after their deposition, these beds have remained
sufficiently soft to receive the impressions of the feet of whatever
animals walked over them, and to preserve them afterwards, in exactly
the same way as such impressions are at this hour preserved on the
shores of the Bay of Fundy and elsewhere. The diagram represents the
track of some gigantic animal, which walked on its hind legs. You see
the series of marks made alternately by the right and by the left foot;
so that, from one impression to the other of the three-toed foot on the
same side, is one stride, and that stride, as we measured it, is six
feet nine inches. I leave you, therefore, to form an impression of the
magnitude of the creature which, as it walked along the ancient shore,
made these impressions.
Of such impressions there are untold thousands upon these sandstones.
Fifty or sixty different kinds have been discovered, and they cover vast
areas. But, up to this present time, not a bone, not a fragment, of any
one of the animals which left these great footmarks has been found; in
fact, the only animal remains which have been met with in all these
deposits, from the time of their discovery to the present day--though
they have been carefully hunted over--is a fragmentary skeleton of one
of the smaller forms. What has become of the bones of all these animals?
You see we are not dealing with little creatures, but with animals that
make a step of six feet nine inches; and their remains must have been
left somewhere. The probability is, that they been dissolved away, and
absolutely lost.
I have had occasion to work out the nature of fossil remains, of which
there was nothing left except casts of the bones, the solid material of
the skeleton having been dissolved out by percolating water. It was a
chance, in this case, that the sandstone happened to be of such a
constitution as to set, and to allow the bones to be afterward dissolved
out, leaving cavities of the exact shape of the bones. Had that
constitution been other than what it was, the bones would have been
dissolved, the layers of sandstone would have fallen together into one
mass, and not the slightest indication that the animal had existed would
have been discoverable.
I know of no more striking evidence than these facts afford, of the
caution which should be used in drawing the conclusion, from the absence
of organic remains in a deposit, that animals or plants did not exist at
the time it was formed. I believe that, with a right understanding of
the doctrine of evolution on the one hand, and a just estimation of the
importance of the imperfection of the geological record on the other,
all difficulty is removed from the kind of evidence to which I have
adverted; and that we are justified in believing that all such cases are
examples of what I have designated negative or indifferent
evidence--that is to say, they in no way directly advance the hypothesis
of evolution, but they are not to be regarded as obstacles in the way of
our belief in that doctrine.
I now pass on to the consideration of those cases which, for reasons
which I will point out to you by and by, are not to be regarded as
demonstrative of the truth of evolution, but which are such as must
exist if evolution be true, and which therefore are, upon the whole,
evidence in favour of the doctrine. If the doctrine of evolution be
true, it follows, that, however diverse the different groups of animals
and of plants may be, they must all, at one time or other, have been
connected by gradational forms; so that, from the highest animals,
whatever they may be, down to the lowest speck of protoplasmic matter in
which life can be manifested, a series of gradations, leading from one
end of the series to the other, either exists or has existed.
Undoubtedly that is a necessary postulate of the doctrine of evolution.
But when we look upon living Nature as it is, we find a totally
different state of things. We find that animals and plants fall into
groups, the different members of which are pretty closely allied
together, but which are separated by definite, larger or smaller, breaks
from other groups. In other words, no intermediate forms which bridge
over these gaps or intervals are, at present, to be met with.
To illustrate what I mean: Let me call your attention to those
vertebrate animals which are most familiar to you, such as mammals,
birds, and reptiles. At the present day, these groups of animals are
perfectly well defined from one another. We know of no animal now living
which, in any sense, is intermediate between the mammal and the bird, or
between the bird and the reptile; but, on the contrary, there are many
very distinct anatomical peculiarities, well-defined marks, by which the
mammal is separated from the bird, and the bird from the reptile. The
distinctions are obvious and striking if you compare the definitions of
these great groups as they now exist.
The same may be said of many of the subordinate groups, or orders, into
which these great classes are divided. At the present time, for example,
there are numerous forms of non-ruminant pachyderms, or what we may call
broadly, the pig tribe, and many varieties of ruminants. These latter
have their definite characteristics, and the former have their
distinguishing peculiarities. But there is nothing that fills up the gap
between the ruminants and the pig tribe. The two are distinct. Such also
is the case in respect of the minor groups of the class of reptiles. The
existing fauna shows us crocodiles, lizards, snakes, and tortoises; but
no connecting link between the crocodile and lizard, nor between the
lizard and snake, nor between the snake and the crocodile, nor between
any two of these groups. They are separated by absolute breaks. If,
then, it could be shown that this state of things had always existed,
the fact would be fatal to the doctrine of evolution. If the
intermediate gradations, which the doctrine of evolution requires to
have existed between these groups, are not to be found anywhere in the
records of the past history of the globe, their absence is a strong and
weighty negative argument against evolution; while, on the other hand,
if such intermediate forms are to be found, that is so much to the good
of evolution; although, for reasons which I will lay before you by and
by, we must be cautious in our estimate of the evidential cogency of
facts of this kind.
It is a very remarkable circumstance that, from the commencement of the
serious study of fossil remains; in fact, from the time when Cuvier
began his brilliant researches upon those found in the quarries of
Montmartre, palæontology has shown what she was going to do in this
matter, and what kind of evidence it lay in her power to produce.
I said just now that, in the existing Fauna, the group of pig-like
animals and the group of ruminants are entirely distinct; but one of the
first of Cuvier's discoveries was an animal which he called the
_Anoplotherium_, and which proved to be, in a great many important
respects, intermediate in character between the pigs, on the one hand,
and the ruminants on the other. Thus research into the history of the
past did, to a certain extent, tend to fill up the breach between the
group of ruminants and the group of pigs. Another remarkable animal
restored by the great French palæontologist, the _Palæotherium_,
similarly tended to connect together animals to all appearance so
different as the rhinoceros, the horse, and the tapir. Subsequent
research has brought to light multitudes of facts of the same order;
and, at the present day, the investigations of such anatomists as
Rütimeyer and Gaudry have tended to fill up, more and more, the gaps in
our existing series of mammals, and to connect groups formerly thought
to be distinct.
But I think it may have an especial interest if, instead of dealing with
these examples, which would require a great deal of tedious osteological
detail, I take the case of birds and reptiles; groups which, at the
present day, are so clearly distinguished from one another that there
are perhaps no classes of animals which, in popular apprehension, are
more completely separated. Existing birds, as you are aware, are covered
with feathers; their anterior extremities, specially and peculiarly
modified, are converted into wings, by the aid of which most of them are
able to fly; they walk upright upon two legs; and these limbs, when they
are considered anatomically, present a great number of exceedingly
remarkable peculiarities, to which I may have occasion to advert
incidentally as I go on, and which are not met with, even approximately,
in any existing forms of reptiles. On the other hand, existing reptiles
have no feathers. They may have naked skins, or be covered with horny
scales, or bony plates, or with both. They possess no wings; they
neither fly by means of their fore-limbs, nor habitually walk upright
upon their hind-limbs; and the bones of their legs present no such
modifications as we find in birds. It is impossible to imagine any two
groups more definitely and distinctly separated, notwithstanding certain
characters which they possess in common.
As we trace the history of birds back in time, we find their remains,
sometimes in great abundance, throughout the whole extent of the
tertiary rocks; but, so far as our present knowledge goes, the birds of
the tertiary rocks retain the same essential characters as the birds of
the present day. In other words, the tertiary birds come within the
definition of the class constituted by existing birds, and are as much
separated from reptiles as existing birds are. Not very long ago no
remains of birds had been found below the tertiary rocks, and I am not
sure but that some persons were prepared to demonstrate that they could
not have existed at an earlier period. But in the course of the last few
years, such remains have been discovered in England; though,
unfortunately, in so imperfect and fragmentary a condition, that it is
impossible to say whether they differed from existing birds in any
essential character or not. In your country the development of the
cretaceous series of rocks is enormous; the conditions under which the
later cretaceous strata have been deposited are highly favourable to the
preservation of organic remains; and the researches, full of labour and
risk, which have been carried on by Professor Marsh in these cretaceous
rocks of Western America, have rewarded him with the discovery of forms
of birds of which we had hitherto no conception. By his kindness, I am
enabled to place before you a restoration of one of these extraordinary
birds, every part of which can be thoroughly justified by the more or
less complete skeletons, in a very perfect state of preservation, which
he has discovered. This _Hesperornis_ (Fig. 3), which measured between
five and six feet in length, is astonishingly like our existing divers
or grebes in a great many respects; so like them indeed that, had the
skeleton of _Hesperornis_ been found in a museum without its skull, it
probably would have been placed in the same group of birds as the divers
and grebes of the present day.[1]
[Illustration: FIG. 3.--HESPERORNIS REGALIS (Marsh).]
But _Hesperornis_ differs from all existing birds, and so far resembles
reptiles, in one important particular--it is provided with teeth. The
long jaws are armed with teeth which have curved crowns and thick roots
(Fig. 4), and are not set in distinct sockets, but are lodged in a
groove. In possessing true teeth, the _Hesperornis_ differs from every
existing bird, and from every bird yet discovered in the tertiary
formations, the tooth-like serrations of the jaws in the _Odontopteryx_
of the London clay being mere processes of the bony substance of the
jaws, and not teeth in the proper sense of the word. In view of the
characteristics of this bird we are therefore obliged to modify the
definitions of the classes of birds and reptiles. Before the discovery
of _Hesperornis_, the definition of the class Aves based upon our
knowledge of existing birds, might have been extended to all birds; it
might have been said that the absence of teeth was characteristic of the
class of birds; but the discovery of an animal which, in every part of
its skeleton, closely agrees with existing birds, and yet possesses
teeth, shows that there were ancient birds which, in respect of
possessing teeth, approached reptiles more nearly than any existing bird
does, and, to that extent, diminishes the _hiatus_ between the two
classes.
[Illustration: FIG. 4.--HESPERORNIS REGALIS (Marsh).
(Side and upper views of half the lower jaw; side and end views of a
vertebra and a separate tooth.)]
The same formation has yielded another bird _Ichthyornis_ (Fig. 5),
which also possesses teeth; but the teeth are situated in distinct
sockets, while those of _Hesperornis_ are not so lodged. The latter also
has such very small, almost rudimentary, wings, that it must have been
chiefly a swimmer and a diver, like a Penguin; while _Ichthyornis_ has
strong wings and no doubt possessed corresponding powers of flight.
_Ichthyornis_ also differed in the fact that its vertebræ have not the
peculiar characters of the vertebræ of existing and of all known
tertiary birds, but were concave at each end. This discovery leads us to
make a further modification in the definition of the group of birds, and
to part with another of the characters by which almost all existing
birds are distinguished from reptiles.
[Illustration: FIG. 5.--ICHTHYORNIS DISPAR (Marsh).
(Side and upper views of half the lower jaw; and side and end views of a
vertebra.)]